Patent application title: COMPOSITES AND METHODS FOR TREATING BONE

Abstract:

A system and method for treating bone abnormalities including vertebral
compression fractures and the like. In one vertebroplasty method, a fill
material is injected under high pressures into cancellous bone wherein
the fill material includes a flowable bone cement component and an
elastomeric polymer component that is carried therein. The elastomer
component can further carry microscale or mesoscale reticulated elements.
Under suitable injection pressures, the elastomeric component ultimately
migrates within the flowable material to alter the apparent viscosity
across the plume of fill material to accomplish multiple functions. For
example, the differential in apparent viscosity across the fill material
creates a broad load-distributing layer within cancellous bone for
applying retraction forces to cortical bone endplates. The differential
in apparent viscosity also transitions into a flow impermeable layer at
the interface of cancellous bone and the flowable material to prevent
extravasasion of the flowable bone cement component.

Claims:

1. A method of treating mammalian bone, comprising:(a) flowing an initial
volume of flowable media into the interior of a bone, the media including
a volume of elastomeric elements; and(b) flowing under pressure an
additional volume of flowable media into the initial volume wherein the
pressure causes a differential apparent viscosity within regions of the
flowable media.

2. The method of claim 1 wherein step (b) causes surface regions of the
flowable media to have substantially higher apparent viscosity than
interior regions of the flowable media.

3. The method of claim 1 wherein step (b) causes surface regions of the
flowable media to be substantially impermeable to flows therethrough.

4. The method of claim 1 wherein step (b) causes surface regions of the
flowable media to apply expansion forces to the bone.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application is a continuation of U.S. application Ser. No.
11/148,973 filed Jun. 9, 2005, which claims benefit of Provisional U.S.
Patent Application No. 60/578,182 filed Jun. 9, 2004, titled Scaffold
Composites and Methods for Treating Abnormalities in Bone, the entire
contents of all of which are hereby incorporated by reference in their
entirety and should be considered a part of this specification.

BACKGROUND OF THE INVENTION

[0002]1. Field of the Invention

[0003]This invention relates to bone implant materials and methods and
more particularly to composite materials including an elastomer component
for treating abnormalities in bones such as compression fractures of
vertebra, necrosis of femurs, joint implants and the like. An exemplary
method includes introducing a flowable composite material into the
interior of a bone wherein increasing pressures result in the elastomer
component causing a differential apparent viscosity within selected
regions across the flowable material to thereby allow controlled
application of forces to the bone for reducing a fracture.

[0004]2. Description of the Related Art

[0005]Osteoporotic fractures are prevalent in the elderly, with an annual
estimate of 1.5 million fractures in the United States alone. These
include 750,000 vertebral compression fractures (VCFs) and 250,000 hip
fractures. The annual cost of osteoporotic fractures in the United States
has been estimated at $13.8 billion: The prevalence of VCFs in women age
50 and older has been estimated at 26%. The prevalence increases with
age, reaching 40% among 80-year-old women. Medical advances aimed at
slowing or arresting bone loss from aging have not provided solutions to
this problem. Further, the affected population will grow steadily as life
expectancy increases. Osteoporosis affects the entire skeleton but most
commonly causes fractures in the spine and hip. Spinal or vertebral
fractures also have serious consequences, with patients suffering from
loss of height, deformity and persistent pain which can significantly
impair mobility and quality of life. Fracture pain usually lasts 4 to 6
weeks, with intense pain at the fracture site. Chronic pain often occurs
when one level is greatly collapsed or multiple levels are collapsed.

[0006]Postmenopausal women are predisposed to fractures, such as in the
vertebrae, due to a decrease in bone mineral density that accompanies
postmenopausal osteoporosis. Osteoporosis is a pathologic state that
literally means "porous bones". Skeletal bones are made up of a thick
cortical shell and a strong inner meshwork, or cancellous bone, of
collagen, calcium salts and other minerals. Cancellous bone is similar to
a honeycomb, with blood vessels and bone marrow in the spaces.
Osteoporosis describes a condition of decreased bone mass that leads to
fragile bones which are at an increased risk for fractures. In an
osteoporotic bone, the sponge-like cancellous bone has pores or voids
that increase in dimension, making the bone very fragile. In young,
healthy bone tissue, bone breakdown occurs continually as the result of
osteoclast activity, but the breakdown is balanced by new bone formation
by osteoblasts. In an elderly patient, bone resorption can surpass bone
formation thus resulting in deterioration of bone density. Osteoporosis
occurs largely without symptoms until a fracture occurs.

[0007]Vertebroplasty and kyphoplasty are recently developed techniques for
treating vertebral compression fractures. Percutaneous vertebroplasty was
first reported by a French group in 1987 for the treatment of painful
hemangiomas. In the 1990's, percutaneous vertebroplasty was extended to
indications including osteoporotic vertebral compression fractures,
traumatic compression fractures, and painful vertebral metastasis. In one
percutaneous vertebroplasty technique, bone cement such as PMMA
(polymethylmethacrylate) is percutaneously injected into a fractured
vertebral body via a trocar and cannula system. The targeted vertebrae
are identified under fluoroscopy. A needle is introduced into the
vertebral body under fluoroscopic control to allow direct visualization.
A transpedicular (through the pedicle of the vertebrae) approach is
typically bilateral but can be done unilaterally. The bilateral
transpedicular approach is typically used because inadequate PMMA infill
is achieved with a unilateral approach.

[0008]In a bilateral approach, approximately 1 to 4 ml of PMMA are
injected on each side of the vertebra. Since the PMMA needs to be forced
into cancellous bone, the technique requires high pressures and fairly
low viscosity cement. Since the cortical bone of the targeted vertebra
may have a recent fracture, there is the potential of PMMA leakage. The
PMMA cement contains radiopaque materials so that when injected under
live fluoroscopy, cement localization and leakage can be observed. The
visualization of PMMA injection and extravasasion are critical to the
technique--and the physician terminates PMMA injection when leakage is
evident. The cement is injected using small syringe-like injectors to
allow the physician to manually control the injection pressures.

[0009]Kyphoplasty is a modification of percutaneous vertebroplasty.
Kyphoplasty involves a preliminary step that comprises the percutaneous
placement of an inflatable balloon tamp in the vertebral body. Inflation
of the balloon creates a cavity in the bone prior to cement injection.
Further, the proponents of percutaneous kyphoplasty have suggested that
high pressure balloon-tamp inflation can at least partially restore
vertebral body height. In kyphoplasty, it has been proposed that PMMA can
be injected at lower pressures into the collapsed vertebra since a cavity
exists to receive the cement--which is not the case in conventional
vertebroplasty.

[0010]The principal indications for any form of vertebroplasty are
osteoporotic vertebral collapse with debilitating pain. Radiography and
computed tomography must be performed in the days preceding treatment to
determine the extent of vertebral collapse, the presence of epidural or
foraminal stenosis caused by bone fragment retropulsion, the presence of
cortical destruction or fracture and the visibility and degree of
involvement of the pedicles, Leakage of PMMA during vertebroplasty can
result in very serious complications including compression of adjacent
structures that necessitate emergency decompressive surgery.

[0011]Leakage or extravasasion of PMMA is a critical issue and can be
divided into paravertebral leakage, venous infiltration, epidural leakage
and intradiscal leakage. The exothermic reaction of PMMA carries
potential catastrophic consequences if thermal damage were to extend to
the dural sac, cord, and nerve roots. Surgical evacuation of leaked
cement in the spinal canal has been reported. It has been found that
leakage of PMMA is related to various clinical factors such as the
vertebral compression pattern, and the extent of the cortical fracture,
bone mineral density, the interval from injury to operation, the amount
of PMMA injected and the location of the injector tip. In one recent
study, close to 50% of vertebroplasty cases resulted in leakage of PMMA
from the vertebral bodies. See Hyun-Woo Do et al., "The Analysis of
Polymethylmethacrylate Leakage after Vertebroplasty for Vertebral Body
Compression Fractures", Jour. of Korean Neurosurg. Soc. Vol. 35, No. 5
(May 2004) pp. 478-82,
(http://www.jkns.or.kr/htm/abstract.asp?no=042004086).

[0012]Another recent study was directed to the incidence of new VCFs
adjacent to the vertebral bodies that were initially treated.
Vertebroplasty patients often return with new pain caused by a new
vertebral body fracture. Leakage of cement into an adjacent disc space
during vertebroplasty increases the risk of a new fracture of adjacent
vertebral bodies. See Am. J. Neuroradiol. 2004 February; 25(2):175-80.
The study found that 58% of vertebral bodies adjacent to a disc with
cement leakage fractured during the follow-up period compared with 12% of
vertebral bodies adjacent to a disc without cement leakage.

[0014]Another disadvantage of PMMA is its inability to undergo
remodeling--and the inability to use the PMMA to deliver osteoinductive
agents, growth factors, chemotherapeutic agent and the like. Yet another
disadvantage of PMMA is the need to add radiopaque agents which lower its
viscosity with unclear consequences on its long-term endurance.

[0015]In both higher pressure cement injection (vertebroplasty) and
balloon-tamped cementing procedures (kyphoplasty), the methods do not
provide for well controlled augmentation of vertebral body height. The
direct injection of bone cement simply follows the path of least
resistance with the fractured bone. The expansion of a balloon also
applies compacting forces along lines of least resistance in the
collapsed cancellous bone. Thus, the reduction of a vertebral compression
fracture is not optimized or controlled in high pressure balloons as
forces of balloon expansion occur in multiple directions.

[0016]In a kyphoplasty procedure, the physician often uses very high
pressures (e.g., up to 200 or 300 psi) to inflate the balloon which first
crushes and compacts cancellous bone. Expansion of the balloon under high
pressures close to cortical bone can fracture the cortical bone, or cause
regional damage to the cortical bone that can result in cortical bone
necrosis. Such cortical bone damage is highly undesirable and results in
weakened cortical endplates.

[0017]Kyphoplasty also does not provide a distraction mechanism capable of
100% vertebral height restoration. Further, the kyphoplasty balloons
under very high pressure typically apply forces to vertebral endplates
within a central region of the cortical bone that may be weak, rather
than distributing forces over the endplate.

[0018]There is a general need to provide systems and methods for use in
treatment of vertebral compression fractures that provide a greater
degree of control over introduction of bone support material, and that
provide better outcomes. Embodiments of the present invention meet one or
more of the above needs, or other needs, and provide several other
advantages in a novel and non-obvious manner.

SUMMARY OF THE INVENTION

[0019]The invention provides systems and method of treating bone
abnormalities including vertebral compression fractures, bone tumors and
cysts, avascular necrosis of the femoral head and the like. In one
embodiment, the invention comprises a bone infill system or implant
system with a fill material that includes a flowable component and an
elastomeric polymer component that is deformable in-situ (FIG. 1A). In
one embodiment, the elastomer component comprises a matrix of base
elastomer and a filler of microscale or mesoscale reticulated elements
(FIG. 1B). The elastomeric component corresponding to the invention
performs multiple functions, for example, (i) forming a load-distributing
structure between a bone fill material or structure and the elastomer
component; (ii) mechanically creating a seal at the interface of
cancellous bone and bone fill material or structure to prevent
extravasasion of a flowable material, (iii) creating a substantially
porous layer around the surface of non-porous bone fill material or
structures and/or (vi) creating an insulative layer around the surface of
an exothermic bone fill material. The elastomer component can be used in
bone support treatments or in treatments to move apart cortical bone
surfaces as in treating vertebral compression fractures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]In the following detailed description, similar reference numerals
are used to depict like elements in the various figures.

[0021]FIG. 1A is a greatly enlarged sectional view of a flowable composite
bone infill material such as PMMA with a volume of elastomeric elements
or particles carried therein.

[0022]FIG. 1B is a greatly enlarged sectional view of an elastomeric
element of FIG. 1A with reticulated elements dispersed within the
elastomer.

[0023]FIG. 2A is a schematic view of a spine segment with a vertebra
having a compression fracture showing a method of the invention wherein a
volume of the flowable media of FIG. 1A is injected under pressure into
cancellous bone in a targeted treatment site.

[0024]FIG. 2B is a schematic view of the spine segment of FIG. 2A showing
the pressurized injection of additional flowable wherein the apparent
viscosity of the media is altered at surface regions of the plume by
outward migration of the elastomeric element to thereby create
flow-impermeable surface regions.

[0025]FIGS. 3A-3B are schematic sectional views of a monolith implant
structure fabricated of the composite elastomeric material of FIG. 1B;
with FIG. 3A illustrating the implant structure introduced into a bore in
a bone.

[0026]FIG. 3B illustrate the elastomeric material of FIG. 3A being
inserted in the bore in the bone.

[0027]FIG. 3C illustrates an interference fit bone screw driven into the
elastomeric material of FIGS. 3A-3B which distributes loads about the
bore in cancellous bone.

[0028]FIG. 4 is a sectional cut-away view of one an implant segment with
multiple layers having different moduli.

[0029]FIGS. 5A-5B show an elastomeric implant with a plurality of
composite regions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0030]FIG. 1A illustrates a cross-sectional view of fill material 4 that
comprises flowable component 5 with elastomeric polymer component 6
dispersed therein. The flowable component or material 5 is an in-situ
hardenable bone cement (e.g., PMMA) that is intermixed with elastomeric
component 6 that comprises a plurality of small elastomeric elements,
such as silicone particles or elements of another biocompatible polymer.
The flowable material 5 and elastomeric elements 6 can be intermixed
prior to introduction into bone or contemporaneous with introduction into
bone from separate channels in an introducer. The elastomeric elements 6
are typically dimensioned to be small enough to allow their passage
within the openings of cancellous bone in a targeted treatment site. In
one embodiment as depicted in FIG. 1B, the elastomeric elements 6
themselves comprise a composite of base elastomer 10A and reticulated,
open-cell scaffold structures indicated at 10B. Such reticulated
open-cell structures can allow for later hone ingrowth into the surface
of the volume of fill material. The term "reticulated" as used herein
describes open-cell structures 10B and means having the appearance of, or
functioning as, a wire-like network or a substantially rigid net-like
structure. The terms reticulated and trabecular are used interchangeably
herein to describe structures having ligaments that bound open cells or
closed cells in the interior of the structure,

[0031]FIG. 2A-2B illustrate a method corresponding to the invention for
use in the treatment of a vertebral compression fracture indicated at 13.
In FIG. 2A, an initial volume of fill material 4 comprising a flowable
bone cement component 5 and intermixed elastomeric elements 6 is injected
under substantial pressure into cancellous bone 14 of the vertebra 15
resulting in plume 18. The fill material 4 is introduced in a unilateral
or bilateral transpedicular approach through cannula 19 as is well known
in the art of vertebroplasty. The fill material 4 propagates within the
openings in cancellous bone and may also follow pre-existing fracture
lines in cancellous bone, for example as may exist following a
compression fracture. FIG. 2B illustrates the same step of injecting fill
material 4 but after a greater volume of material has been introduced
resulting in plume 18 of fill material being larger and engaging the
cortical bone endplates. In the high pressure injection of such a
composite fill material, the elastomeric elements 6 migrate toward a
surface region 20 of the plume 18 and create a differential in the
apparent viscosity of the flowable material across the volume or plume.
The term "apparent viscosity" is used herein to describe the flow
characteristics of the combination of flowable component 5 and intermixed
elastomeric elements 6. As the injection pressures and the resistance to
inflows of fill material increase, the accumulation of elastomeric
elements 6 about surface region 20 also increases. The elastomeric
elements 6 can additionally deform and ultimately the pressures cause
elastomeric elements 6 to form in-situ a substantially flow-impermeable
surface region 20. As the surface region becomes substantially
impermeable to flows or extravasasion therethrough of flowable component
5, continued injection of fill material will elastically expand the
surface regions and apply expansion forces to the bone. In a vertebral
body as in FIG. 2B, the expansion pressures can expand cancellous bone 14
in which the flowable material 4 has flowed and apply retraction forces
to the cortical bone endplates to at least partly reduce a vertebral
fracture.

[0032]In general, an exemplary method corresponding to the invention for
treating mammalian bone comprises the following: (a) flowing an initial
volume of flowable media into the interior of a bone wherein the media
includes a volume of elastomeric elements, and (b) flowing under pressure
increasing volumes of the flowable media wherein injection pressures
causes a differential apparent viscosity within selected regions across
the flowable media. The method further includes causing surface regions
20 of the plume 18 of flowable media to be substantially impermeable to
flows therethrough (FIG. 2B), The method includes allowing an in-situ
polymerizable component of the flowable media to harden to thereby
support expanded cancellous bone and to maintain retracted cortical bone
in an altered position.

[0033]In another embodiment, the fill material 4 described above includes
an elastomer filler composite 6 that carries microscale or mesoscale
reticulated elements 10B (FIG. 1B). As the elastomer elements 6 aggregate
about surface region 20 of the plume 18, the reticulated material is
proximate to bone and can thus allow for subsequent bone ingrowth. In
addition, elastomer elements 6 and surface region 20 create an insulative
layer that prevents or moderates heating of the bone external to surface
region 20 from an exothermic reaction of a typical bone cement used as
flowable component 5 that is interior of surface region 20.

[0034]In any embodiment, elastomer composite elements 6 can carry
radiosensitive and magnetic-sensitive fillers for cooperating with an RF
source or an inductive heating source for elevating the polymer to a
targeted temperature. Alternatively, the polymeric composition can be
substantially transparent or substantially translucent and carry
chromophores for cooperating with a light source introduced with the
material for heating to material to a selected temperature for increasing
the modulus of the material. Thus, such methods of heating surface
regions 20 (FIG. 2B) in which the elastomer composite elements 6 have
aggregated will cause accelerated heating of adjacent interior regions of
flowable component 5. This system can be used to selectively polymerize
regions of flowable media 5 adjacent the surface region 20. By this
means, the peripheral portions of plume 18 interior of, and within, the
aggregated elastomeric elements, can be formed into a flow-impermeable
layer.

[0035]The reticulated structures 10B as in FIG. 1B define a mean cross
section which can be expressed in microns. In preferred embodiments, the
cells are bounded by polyhedral faces, typically pentagonal or hexagonal,
that are formed with five or six ligaments 15. The cell dimension is
selected for enhancing tissue ingrowth, and mean cell cross-sections can
range between 10 microns and 200 microns; and more preferably ranges
between 20 microns and 100 microns. Such reticulated materials and
structures are available from ERG Materials and Aerospace Corp., 900
Stanford Avenue, Oakland Calif. 94608 and Porvair Advanced Materials,
Inc., 700 Shepherd Street, Hendersonville N.C. 28792, and are more fully
described in U.S. patent application Ser. No. 11/146,891, filed Jun. 7,
2005 titled Implants and Methods for Treating Bone, the contents of which
are incorporated herein by this reference in their entirety and should be
considered a part of this specification.

[0036]Referring back to FIGS. 1A and 1B, the elastomeric composition
comprises any biocompatible polymer having an elastic modulus ranging
between about 10 MPa and 1 KPa. The polymer can be a foam, or a shape
memory polymer (SMP) that releases stored energy after heating and moving
from a compacted temporary shape to an expanded memory shape. A
description of suitable shape memory polymers is described in U.S. patent
application Ser. No. 10/837,858 titled Orthopedic Implants, Methods of
Use and Methods of Fabrication filed May 3, 2004, the contents of which
are incorporated herein by this reference in their entirety and should be
considered a part of this specification. In a preferred embodiment, the
elastomer elements 5 are at least one of bioerodible, bioabsorbable or
bioexcretable.

[0037]FIGS. 3A-3C illustrate an alternative embodiment of the invention
wherein the composite of an elastomer 10A and reticulated elements 10B
(FIG. 1B) is formed into exemplary implant body 40A. In FIGS. 3A and 3B,
implant 40A is fabricated by molding in a suitable dimension for
introduction into bore 25 in a bone, indicated as cancellous bone 26 and
a cortical bone surface 28. FIG. 3C illustrates that implant 40A can have
an optional channel or opening 44 for receiving or guiding the
positioning of fill material 48 comprising a threaded implant. In FIG.
3C, it can be seen that a threaded implant 48 can be screwed into the
implant wherein the elastomeric implant 40A and reticulated elements 10B
dispersed therein are compressed to form an interference fit between the
bone and implant member 40A. Of particular interest, the insertion of the
threaded implant 48 causes self-adjustment of the distribution, location
and orientation of the reticulated elements 10B within the elastomer
matrix, thus optimally self-distributing loads between the implant 48 and
the bone. In the prior art, a threaded implant would engage the bone
highest engagement pressures generally about the apex of the threads. In
the system as in FIG. 3C, the engagement forces would be distributed
about all surfaces of threaded implant 48--which also preferably has a
surface region that is reticulated, roughened or porous.

[0038]FIG. 4 illustrates another exemplary implant 40B that is fabricated
of an elastomer composite. In this embodiment, the composite body has at
least two layers 50a and 50b that are polymer matrices that carry
reticulated elements having different parameters (density, cell
dimensions etc.) to provide different elastic moduli. The scope of the
invention thus encompasses an implant structure 40B that has a gradient
modulus for transitioning from an interlace with cortical bone 55 to the
interface with a rigid member 48 which is needed in various implants and
reconstructions, such as in hip implants.

[0039]In another embodiment depicted in FIGS. 5A and 5B, the elastomeric
composite implant 60 can be configured with a plurality of composite
regions 62a and 62b that provide variations or gradients in material
properties for enhancing implant fixation in bone 64. In FIG. 5B, it can
be seen that regions 62a of the composite are deformable but more rigid
than the adjacent regions 62b. Thus, the higher modulus regions will be
forced outward more into the bone that other regions 62b upon insertion
of bone screw 68. The scope of the invention encompasses varying all the
obvious properties of different regions of the composite to achieve the
desired regional variations or gradients, and include adjusting the: (i)
density of ligaments of the reticulated elements dispersed in the matrix;
(ii) the overall shape, dimensions and orientations of the reticulated
elements; (iii) the pore size of the reticulated elements; (iv) the
modulus, deformability and material of the reticulated elements; (v) the
percentage volume of reticulated elements in the matrix, (vi) the
properties media carried in the pores of the reticulated elements, and
(vii) the modulus and other properties of the polymer base material 10A
(FIG. 1B).

[0040]The above-described embodiments describe elastomer composites that
cooperate with fill materials to control properties of the interface
between fill material and bone. The scope of the invention extends to
elastomer composites as in FIGS. 2A-2B, 3A-3C and 4 that are introduced
into bone wherein a base polymer can be elevated to a transition
temperature so that the composite then adjusts its orientation. Upon
cooling, the elastomer composite can then freeze in a particular form. In
such embodiments, it is preferred that reticulated elements in the
composite have" varied shapes for non-slip engagement between such
elements to thereby increase the modulus of the material. In an exemplary
embodiment, the polymeric composition has a transition temperature in the
range of 40° C. to 120° C.; and preferably in the range of
40° C. to 80° C. The transition temperature is a glass
transition temperature or a melt temperature. Again, the polymeric matrix
can carry radiosensitive or magnetic-sensitive fillers for cooperating
with an RF source or an inductive heating source for elevating the
polymer to a targeted temperature. Alternatively, the polymeric
composition can be substantially transparent or substantially translucent
and carry chromophores for cooperating with a light source for heating to
material to a selected temperature for elevating the composition to a
transition temperature.

[0041]In any embodiment, the fill materials or implants can further carry
a radiopaque or radiovisible composition if the material of the
reticulated elements is not radiovisible.

[0042]In any embodiment, the fill materials or implants can carry any
pharmacological agent or any of the following: antibiotics, cortical bone
material, synthetic cortical replacement material, demineralized bone
material, autograft and allograft materials. The implant body also can
include drugs and agents for inducing bone growth, such as bone
morphogenic protein (BMP). The implants can carry the pharmacological
agents for immediate or timed release.

[0043]The above description of the invention intended to be illustrative
and not exhaustive. A number of variations and alternatives will be
apparent to one having ordinary skills in the art. Such alternatives and
variations are intended to be included within the scope of the claims.
Particular features that are presented in dependent claims can be
combined and fall within the scope of the invention. The invention also
encompasses embodiments as if dependent claims were alternatively written
in a multiple dependent claim format with reference to other independent
claims.